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AD1672AP View Datasheet(PDF) - Analog Devices

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AD1672AP Datasheet PDF : 20 Pages
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AD1672
REFERENCE INPUT
The nominal reference input is 2.5 V, taken with respect to
REFCOM. The reference input pin (REFIN) can be connected
to the reference output pin (REFOUT) or a standard external
+2.5 Volt reference can be selected to meet specific system re-
quirements. Figure 17 shows the AD1672’s REFIN being
driven from an external precision voltage reference such as the
REF192. If an external reference is used, REFOUT can be ef-
fectively disabled by tying it directly to the positive supply volt-
age VCC. Note that if the reference is changed during a conversion,
all three conversions in the pipeline will be invalidated.
The nominal 2.5 V reference input is rescaled to 1.25 V via a
resistor divider network as shown in Figure 18. The nominal
resistance of this resistor network is 8 k. Table IV summa-
rizes various 2.5 V references for use with the AD1672 (see
REFERENCE OUTPUT section).
Table IV. Suitable 2.5 Volt References
Drift
(ppm/؇C)
Initial Accuracy
%
REF43B
REF192E
AD780B
10 (max)
0.1
5 (max)
0.08
3 (max)
0.04
+VCC
+VCC
10µF
0.1µF
AD780
VS
GND OUTPUT
REFOUT
AD1672
REFIN
1µF
Figure 17. Circuit using AD780 Optional +2.5 V
Reference Input Circuit
REFERENCE NOISE REDUCTION
Figure 18 shows the proper connection of two external 1 µF
capacitors, CNR1 and CNR2, associated with the two noise re-
duction nodes NCOMP1 and NCOMP2 (Pins 25 and 26) of
the AD1672. Although these noise reduction capacitors, CNR, are
not required for stability, the most optimum noise and distortion per-
formance will be attained with a 1 µF ceramic capacitor connected
from each Noise Reduction node to ACOM. CNR1 is associated with
the output of the DAC amp and is required for optimum dis-
tortion performance and SHA settling time. It provides a low
source impedance for signal-dependent, transient currents asso-
ciated with this node. CNR2 reduces the noise contribution from
the reference buffer amplifier.
1µF
20 REFOUT
AD1672
CNR2
1µF
CNR1
1µF
REFIN
4k
23
REFCOM 4k
24
NCOMP2
25
NCOMP1
26
REF
AMP
DAC
AMP
Figure 18. Reference Noise Reduction
REFERENCE OUTPUT
The AD1672 includes an onboard +2.5 V curvature compen-
sated bandgap reference that has been laser trimmed for both
absolute value and temperature drift. The reference must be
externally compensated with a capacitor of 1 µF or greater from
REFOUT to REFCOM.
The reference is specified to provide an additional load current
up to 500 µA. For large dynamic loads and/or external loads
greater than 500 µA, an external buffer amplifier or pull-up re-
sistor is required. If a pull-up resistor is used, its value should
be selected such that REFOUT will not be required to sink any
current under all operating conditions. Figure 19 illustrates the
load regulation of the reference. The power supply rejection of
the reference is better than 0.2 % of FSR at dc.
2.500
2.499
2.498
2.497
2.496
2.495
2.494
2.493
2.492
2.491
2.490
0
100 200 300 400 500 600 700 800 900 1000
LOAD CURRENT – µA
Figure 19. Reference Output Voltage vs. Load Current
DIGITAL OUTPUTS
The AD1672 output data is presented in positive true straight
binary for both the 0 V to 2.5 V and 0 V to 5 V unipolar input
ranges and positive true offset binary for the ± 2.5 V bipolar
range. Table V indicates the output data formats for the three
analog input ranges. Users requiring twos complement encoding
for the ± 2.5 V range may simply invert the MSB (Pin 13).
Table V. Output Data Format
Input Voltage Range +FS (V) –FS (V)
0 to +2.5
0 to +5.0
–2.5 to +2.5
+2.5
0.0
+5.0
0.0
+2.5
–2.5
LSB (mV)
0.610
1.22
1.22
OTR
1
0
0
DATA OUTPUTS
1111 111 1111
1111 1111 1111
1111 1111 1110
OTR
–FS+1/2 LSB
+FS –1 1/2 LSB
0
0000 0000 0001
0
0000 0000 0000
1
0000 0000 0000
–FS
–FS –1/2 LSB
+FS
+FS –1/2 LSB
Figure 20. Output Data Format
REV. 0
–11–
 

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